5-Chloro-N-({(5S)-2-oxo-3-[4-(5,6-dihydro-4H-[1,2,4]triazin-1-yl)phenyl]-1,3-oxazolidin-5-yl}-methyl)thiophene-2-carboxamide of formula (A) has been known as an inhibitor of blood coagulation factor Xa and used for treating and preventing thrombosis, myocardial infarction, arteriosclerosis, inflammation, stroke, angina pectoris, recurrent stricture after angioplasty, and thromboembolism such as intermittent claudication.

Korea Patent No. 2008-64178, whose application has been filed by the present invetors, discloses a use of the compound as an inhibitor of blood coagulation factor Xa and a preparation method thereof. The preparation method comprises the step of preparing a cyclic amidrazone starting from 4-nitroaniline, as shown in reaction scheme 1:

Specifically, the cyclic amidrazone (A) is prepared by the steps of: preparing the compound (B) using 4-nitroaniline; treating the compound (B) with a t-butoxycarbonyl amine protecting group to prepare the compound (C); introducing a nitroso group into the compound (C) using NaNO2, followed by reduction using zinc to prepare the compound (D); and treating the compound (D) successively with hydrochloric acid and an ortho-formate.
However, the above preparation method is complicated and gives a low yield of the compound (A) (e.g., a total yield of 9%), and it also requires the use of a column chromatography purification step, which limits mass production of the cyclic amidrazone. In particular, the step for preparing the compound (D) from the compound (C) is required to use a harmful heavy metal-containg materal such as zinc amalgam which gives an unsatisfactorily low yield, and the isolation step of the compound (D) does not proceed easily.